New High-Pressure Form of the Negative Thermal Expansion Materials Zirconium Molybdate and Hafnium Molybdate

High-pressure X-ray diffraction experiments on cubic ZrMo 2 O 8 have shown that the material undergoes a first-order phase transition involving an 11% volume decrease between 0.7 and 2.0 GPa under quasi-hydrostatic conditions. The transition is reversible upon decompression, but shows considerable hysteresis. Similar behavior was observed for cubic HfMo 2 O 8 . Under nonhydrostatic conditions, the cubic materials start to amorphize above 0.3 GPa and can be converted to the monoclinic polymorphs when heated under pressure. This is the first time that monoclinic HfMo 2 O 8 was observed.